Add unit tests for `services/yjs`

[kulmann]

Initial Checklist

• I understand this is a user story and questions should be posted in the Community Discussions
• I searched issues and couldn’t find anything (or linked relevant results below)

User Story

As a developer, I want services/yjs to be covered by unit tests so that I can change the collaboration logic without silently breaking session handling.

User Value

Quality, maintainability

Acceptance Criteria

• Tests exist for: provider setup and teardown, room name generation, awareness handling, dirty state handling, hydration, recovery (canSupplyRecoveryContent), etag comparison, authentication/token handling and reconnect behaviour
• Tests run in CI on every PR